The signs have bugged so whenever you place them or any existing ones are just text and no wooden parts.
Do you have any mods installed that could affect this? (If you don't know just list all your mods and I will check for you)
If you hang around in the mining world frequently, your client could be having graphical issues. (I've experienced this too, but only when being in the mining world for an extended period of time.) This is your client and cannot be fixed server-side.
